Electrical Conductivity and Hardness Property of CNTs/epoxy Nanocomposites

Hendra Suherman 1,3 a, Jaafar Sahari 1,2, b and Abu Bakar Sulong 1,2,c

1Fuel Cell Institute, Universiti Kebangsaan Malaysia, 43600 UKM Bangi,Selangor, Malaysia

2Department of Mechanical and Materials Engineering, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia

3Department of Mechanical Engineering, Universitas Bung Hatta, 25143 Padang, West Sumatera, Indonesia

ahenmeubh@yahoo.com, bjaafar@eng.ukm.my, cabubakar@eng.ukm.my

Keywords: Carbon nanotubes, Electrical conductivity, Hardness, Nanocomposites

Abstract. This study investigates the effect of carbon nanotubes (CNTs) as conductive fillers and

epoxy resin as matrix on the electrical conductivity and hardness property. The different CNTs

weight percentage (0 ~ 10 wt.%) were added into the epoxy resin. The dispersion of CNTs in epoxy

resin was conducted by high speed mixer through mechanical shearing mechanism. The mixture of

CNTs/epoxy was poured into the mold and compression molding was conducted for fabrication of

CNTs/epoxy nanocomposites. The electrical conductivity and hardness of CNTs/epoxy

nanocomposites by several of CNTs loading concentration were measured by the four point probe

and dynamic ultra micro hardness tester. Agglomeration of CNTs in epoxy matrix was observed on

fractured surface by scanning electron microscopic. Non conductive epoxy polymer becomes

conductor as addition of CNTs. Electrical conductivity of CNTs/epoxy nanocomposites were

increased with increasing of CNTs loading concentration. Hardness property of CNTs/epoxy

nanocomposites ware reached the highest value at 5 wt.%, and then it was decreased.

Introduction

Since Iijima discovered the structure of carbon nanotubes (CNTs)[1], many properties of CNTs

have been investigated such as physical, electrical, mechanical, thermal and magnetic properties

[2,3]. Treacy et al.[4] used a transmission electron microscope to measure the amplitude of CNTs’

intrinsic thermal vibrations and obtained the average value of Young’s modulus of multi-walled

carbon nanotubes (MWNTs) to be 1.8 TPa. Falvo et al. [5] showed that MWNTs could be bent and

buckled repeatedly under large strain without failure using the tip of an atomic force microscope.

Xie et al.[6] used the chemical vapor deposition method to synthesize aligned carbon nanotubes

(CNTs) and the average Young’s modulus and tensile strength of CNTs were found as 0.45 TPa and

3.6 GPa, respectively. Demczyk et al. [7] conducted pulling and bending tests on carbon nanotubes

in situ in a transmission electron microscope and obtained the tensile strength of 0.15 TPa and

Young’s modulus of 0.8 TPa. Yu et al.[8] reported that a multi-walled nanotube (MWNT) had a

Young’s modulus of 270–950 GPa; while a single-walled nanotube (SWNT) had aYoung’s

modulus of 1–1.2 TPa [9]. There have been many attempts to achieve incorporating nanoparticles,

such as metal-oxide nanoparticles [10-13], nanoclays [14-17], carbon nanofibers [18,19], graphite

nanoplates [20], and carbon nanotubes [21-23], into the polymeric matrix of conventional fiber-

reinforced composites by impregnating dry fiber preform with nanocomposite matrix [10-21]. The

compressive strength of the nano-reinforced composites showed enhancements on the order of 15–

36% [16, 18, 20]. The electrical conductivity of polymers can be adjusted with various CNTs

contents. The CNT/polymer can be used as antistatic material (109

Ω/square), electrostatic discharge

(ESD) material (106 Ω/square) or used as shielding materials for electromagnetic interference (EMI)

or radio frequency interference (RFI) (104 Ω/square)[24]. In the case of polymer matrix, most

papers dealt with relatively brittle and rigid matrix, such as cured epoxy resin. And often, only

mechanical properties [25-26] or physical properties [27] were treated. In this paper, both of the

electrical and hardness properties of CNTc/epoxy nanocomposites were investigated.

Advanced Materials Research Vol. 701 (2013) pp 197-201Online available since 2013/May/27 at www.scientific.net© (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.701.197

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Experimental Materials. The matrix used in this study was a 635 epoxy resin with viscosity 6 poise purchased from US composites. Low viscosity of epoxy resin was choosing to give better wetting condition with fillers. The conductive filler was multi-walled carbon nanotubes (MWNTs) NC7000 purchased from Nanocyl, Belgium. The average diameter is 9.5 nm, and the length is 1.5 µm, with purity ≥90%, reported by manufacturer. Preparation of CNTs/epoxy nanocomposites. The epoxy and curing agent were poured into the container with ratio 3:1 in weight percentage wt.%. The mixture was mixed using the mechanical mixer at 800 rpm for 20s. MWNTs with different loading concentration (2.5, 5, 7.5 and 10 wt.%) were added into epoxy and curing agent mixture, which was mixed again using mechanical mixer at 800 rpm for 3 min. The CNTs/epoxy nanocomposites were fabricated through hot pressing process by 20 Ton hot press machine. CNTs/Epoxy resin/curing agent mixture were poured into the mould. The mould has been pressed at 80

oC for 2 h.

Electrical conductivity. Electrical conductivity of the CNTs/epoxy nanocomposites were measured by Jandel multi height four-point probe combined with a RM3 test unit, which had a constant-current source and digital voltmeter. This technique is measured a sheet resistance in the range from 1mΩ/sq to 5×10

8 Ω/sq and a volume resistivity range from 10

-3 to 10

6 Ω.cm in the plane direction

of specimen. Morphology of fracture surface. The fracture surface morphology of the CNTs/epoxy nanocomposites were investigated with a variable-pressure scanning electron microscope (VPSEM, Model LEO 1450VP) at an accelerating voltage of 20 kV. Measurement of hardness property. The hardness property of CNTs/epoxy nanocomposites were performed using a Dynamic ultra micro hardness tester using a Vickers typed diamond indenter. Dwell load is 0.98 N and dwell is 10 sec was used as test parameter. Three times measurement were performed for each specimen. Before each test, the surfaces of specimens were cleaned with soap to remove oil layer and wiped with tissue.

Results and Discussion Electrical Conductivity. The electrical conductivity of epoxy nanocomposites as function of CNTs (conductive fillers) and loading concentration in weight percentage are shown in Fig. 1. Non-conductive material of pure epoxy polymer becomes conductive as incorporated CNTs. At low loading concentration of CNTs at 2.5 wt.%, CNTs/epoxy nanocomposites already showed improvement of electrical conductivity. The electrical conductivity of CNTs/epoxy nanocomposites are increased with increasing of CNTs loading concentration. The highest electrical conductivity of CNTs/epoxy nanocomposites were reached 2.3 S/cm at 10 wt.% of CNTs. This phenomenon shows that the CNTs as conductive filler acts as the transfer medium for electrons within polymer matrix hence the electrical conductivity of the CNTs/epoxy nanocomposites would be increased [28,29].

All results gave electrical conductivity exceeding 10-8 S/cm, therefore these specimens able to dissipate of electrostatic charges for usage in various applications.

0.0 2.5 5.0 7.5 10.0 12.5

0.5

1.0

1.5

2.0

2.5

Ele

ctri

cal

Cond

uct

ivit

y (

S/c

m)

Content of CNTs in (wt.%) Fig.1. Effect of the weight percentage on the electrical conductivity of CNTs/epoxy

nanocomposites.

198 Key Engineering Materials III

Effect of CNTs on hardness property. The hardness property of CNTs/epoxy nanocomposites as a

function of CNTs loading concentration are shown in Fig. 2. Hardness property of pure epoxy

polymer is 11.28 HV.

-2.5 0.0 2.5 5.0 7.5 10.0 12.58

10

12

14

16

18

20

22

24

Har

dnes

s p

roper

ty (

HV

)

Content of CNTs in (wt. %)

Fig. 2. Effect of the weight percentage on the hardness property of CNTs/epoxy nanocomposites.

Additions of CNTs give slightly increase (6%) of hardness property at 2.5 wt.% CNTs in epoxy

composite. Then, hardness property of CNTs/epoxy nanocomposites were significantly increase

(95%) at 5 wt.% CNTs in epoxy composite. However, after 5 wt.% CNTs addition, hardness

property of CNTs/epoxy nanocomposites decreased sharply until similar to hardness value of pure

epoxy polymer at 10 wt.% of CNTs. The decreasing hardness property above 5 wt.% can be

attributed to the agglomeration of CNTs at the higher loading concentration and lack of matrix

(epoxy resin) to bind the CNTs as conductive fillers, as shown in Fig. 2 [29-30]

Fracture Surface of CNTs/epoxy nanocomposites. Scanning electron microscopic (SEM) image

of the CNTs/epoxy nanocomposite specimen is given in Fig.3. Significant CNTs agglomeration can

be observed on the fracture surfaces, which play a dominant role as electrical conductive pathway in

polymer matrix. However, CNTs used in this study is as produced MWCNTs which easy to

entangle and form agglomeration due to high aspect ratio of individual MWNTs. Fig. 4, (a) and (b)

show magnification of Fig. 3. CNTs observed to be embedded in polymer matrix and some had

been pulled out from polymer matrix. Fig. 4, show that CNTs has moderate wetting property with

epoxy matrix. Further study on increasing dispersion quality and enhancement interfacial bonding

force between CNTs and polymer matrix should be carrying out.

Figure 3. SEM fracture surfaced of CNTs epoxy nanocomposite

Agglomeration

occurs

Advanced Materials Research Vol. 701 199

(a) (b)

Figure 4. Magnification of Fig. 3. (a) lower magnification, and (b) higher magnification.

Summary

Investigation of electrical conductivity and hardness property were conducted on CNT/epoxy

nanocomposites fabricated by the compression molding. Incorporation of CNTs on the polymer

matrix make non-conductive polymer becomes a conductive material. Electrical conductivity of

CNTs/epoxy nanocomposites were increased by increasing loading concentration of CNTs. As

produced CNTs found not dispersed well in epoxy matrix, where conductive pathway network can’t

be form through the CNTs/epoxy nanocomposites. At 5 wt.% addition of CNTs gave the highest

hardness property. Decreasing of hardness property of CNTs/epoxy nanocomposites after 5 wt.%

can be explained by agglomeration of CNTs cause degradation of mechanical property of

CNTs/epoxy nanocomposites. Further research should be conduct, in order to increase dispersion

and forming electrical conductive pathway through the CNTs/epoxy nanocomposites.

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